Optical transmission device and optical module

An optical transmission device includes: an optical modulation package that includes a plurality of terminals that are exposed to an outside from a front surface thereof, and performs optical modulation on electrical signals input from the terminals; a flexible board that includes: an electrode arrangement part on which electrodes respectively connected to the terminals are arranged side by side to transmit the electrical signals to the terminals; and an arm part that projects more outward than an electrode located on an end of the electrode arrangement part and extends toward the optical modulation package to be locked to the optical modulation package; and an optical transmission member that transmits an optical signal obtained by the optical modulation performed by the optical modulation package.